@Rajat
You are mistaken. Registration of the property that is bought is madatory for registration. Registration is to be done within 4 months of the sale of the property as per the ruling of the Honourable Supreme Court. If not then you would have to pay a fine. Also as long as the property is not registered in your name, the property would still belong to the builder as there is not legal proof of your ownership.
You should talk to a lawyer to consult on how to get your house registered. You could also use this very helpful website to get help. Log on to www.lawkonect.com to get detailed written replies to all your legal queries, from practising lawyers. You can also get all your documents made from this website. Being online it is very convenient.
Regards